Two passes beat one clever pass

Two passes beat one clever pass. A surprising number of "hard" array tasks decompose into: pass 1 computes a summary (max, sum, count), pass 2 applies it. The exam's favorite: second distinct maximum. One pass is possible but error-prone (three branches: bigger than max, between max and second, equal to max must be ignored); two passes — find max, then find max excluding it — is nearly mechanical. Under time pressure, mechanical wins.

Equality vs identity for objects: == on Strings/Integers compares references. Use .equals(...) unless both sides are int. The mode problem below is exactly where this bites: comparing list.get(j) == v on Integer objects can pass small-cache tests and fail larger ones — always .equals.

Boundary table for any array scan: empty → spec-defined value (often -1 or a boolean), one element → is it its own answer?, all equal → tie behavior defined by the spec, negatives present → does your initialization survive?